move ui specific note properties to a ui object within each note

all properties that are specific only to ui, like collapsed states,
visible states, etc. are now kept in a 'ui' property of each note.
this way, we can prevent the cluttering of each note object. things
like 'sharing' and 'tags' will not be part of this new property
because although they play a role in ui, they are not solely there
for ui purposes.